In forced air heating and air conditioning systems it is desirable to employ an air filter to cleanse the air of dust, dirt, pollutants, and other airborne particles. Typically, this is done by installing an air filter in the intake duct so that the intake air must flow through the air filter. The air filter may be formed of screening, a fiberglass batt, an electrostatic panel or the like, depending upon environmental conditions and the degree to which it is desired to clean the air. To facilitate ease of service and replacement, the air filter is usually housed in a mounting assembly interposed between the air return duct and the air intake of the furnace or air conditioning unit. The housing generally has an opening on one side to permit inserting the filter and sliding it through the housing across the air flow path. The installation opening is often merely a slot in the housing of the filter mounting assembly and frequently is not even closed by a door.
Generally speaking, the cleaner it is desired for the air to be, the more efficient the air filter unit must be. Thus, panels made up of a batt of fiberglass fibers typically remove more dirt and dust than simple screen filters, which are only effective to trap larger size particles. Properly installed electrostatically charged panels or electronic panels are even more efficient and may remove more than 90% of the airborne pollutants from the air, including mold, spores, smoke, and bacteria. Filter efficiency, however, is often adversely affected by the mounting arrangement and the need to provide for easy and inexpensive access for filter installation and replacement. Accordingly, conventional filter housings with simple side or top openings tend to admit unfiltered air downstream of the filter panel. Moreover, in order to slide the filter panel through the housing, the mounting must provide for a relatively loose fit to prevent binding. This, in turn, allows for a substantial amount of air leakage around the edges of the filter panel rather than causing all of the air to flow through the filter medium.
Conventional filter mounting assemblies are also constructed such that the installation opening and housing can only accommodate filter panels of a standard, predetermined thickness. As a result, if a different thickness filter panel is desired, a refurbishment of the existing filter mounting assembly is usually required. For instance, if a thicker-than-standard filter panel is used, the width of the existing housing and installation opening must be enlarged so that the thicker panel may be received. On the other hand, if a thinner-than-standard filter panel is used, the existing housing does not necessarily have to be modified. Nevertheless, an adjustment is generally advisable--otherwise the fit between the filter panel and the housing will be extremely loose, a substantial amount of air will bypass the filter, and there will be a significant drop in filter efficiency.